Treatment systems and techniques for removing contaminants from contaminated gas have been developed in the past. Some of these treatment systems and techniques include treatments using a photocatalytic process. Common photocatalytic treatment methods typically make use of a technique by which a photocatalyst is bonded or fixed to a substrate or a fluidized bed reactor. Photocatalytic reactions are then caused by irradiating electromagnetic radiation such as ultraviolet light on the fixed photocatalyst so as to activate it. Resulting photocatalytic reactions bring about destruction of contaminants, such as volatile organic contaminants or other biologically harmful compounds that are in close proximity to the activated photocatalyst.
Several significant problems may be encountered when using known photocatalytic air treatment systems and techniques. One problem is the need to maximize "mass transfer," which is the number of collisions between contaminants and an activated photocatalyst. Photocatalytic destruction occurs only at the surface of the photocatalyst where the photocatalyst contacts the contaminants. If an insufficient level of mass transfer occurs, contaminants will pass directly through the treatment system without being subjected to the photocatalyst. Consequently, known photocatalytic air treatment systems are significantly oversized to overcome mass transfer limitations, which reduces the cost effectiveness of such systems.
Another known problem with known photocatalytic air treatment systems is fouling. Humid air streams may contain foulants that oxidize and/or collect on the surface of the photocatalyst. Coating or covering the photocatalyst with foulants therefore substantially lowers the efficiency of the photocatalyst due to a reduction in the effective surface area of the photocatalyst available for irradiation. To prevent the detrimental effects that may be brought about by fouling, the photocatalyst must be frequently cleaned or replaced. This is a time consuming and thus expensive procedure. In addition, bonding or affixing a photocatalyst to a substrate or bed reactor, and maintaining the photocatalyst immobilized, diminishes the efficiency and effectiveness of the photocatalyst. For example, the crystal structure of a photocatalyst is typically undesirably altered when a photocatalyst is heated in the course of bonding the photocatalyst to a substrate. It is also undesirable to replace an already bonded or affixed photocatalyst since such a replacement is a time consuming and expense procedure.
With known photocatalytic air treatment methods, safety issues arise when explosive ratios of contaminants exist in a given air stream. This follows since direct photocatalytic treatment can potentially ignite the explosive contaminants due to temperature elevation or the ignition source from an irradiation element.